asp.net - Unable to determine a valid ordering error when using self referencing -

i have schema :

category entity have reference via parentcategoryid field.parentcategoryid field nullable.i use database first way.this generated code entity :

public partial class category {     public category()     {         this.category1 = new hashset<category>();         this.news = new hashset<news>();     }      public int categoryid { get; set; }     public string name { get; set; }     public nullable<int> parentcategoryid { get; set; }      public virtual icollection<category> category1 { get; set; }     public virtual category category2 { get; set; }     public virtual icollection<news> news { get; set; } } 

when insert category parentcategoryid field nullable,every thing ok,but when select value parentcategoryidi exception:

unable determine valid ordering dependent operations. dependencies may exist due foreign key constraints, model requirements, or store-generated values 

i had category pk=0 , when selected parent got mentioned error.after removing category problem resolved.